FIG. 1 depicts a schematic diagram of a telecommunications system in the prior art. Telecommunications system 100 comprises:                i. telecommunications network 101,        ii. network telecommunications terminals 102 and 103,        iii. private branch exchange 105,        iv. on-premises telecommunications terminals 106 and 107,        v. mobile switching center 110,        vi. cellular network 111, and        vii. cellular telecommunications terminals 112 and 113, all of which are interconnected as shown.        
Telecommunications network 101 is one of multiple, telecommunications networks that are represented in FIG. 1. Telecommunications network 101 comprises the Public Switched Telephone Network, which is a complex of telecommunications equipment that is owned and operated by different entities throughout the World. In the United States of America, for example, the Public Switched Telephone Network (or “PSTN”)comprises an address space that is defined by ten digits, and, therefore, comprises 10 billion unique addresses or “telephone numbers.” The public switched telephone networks in other countries are similar.
Network 101 interconnects the other telecommunications networks, which include: (i) the enterprise network supported by private branch exchange 105, and (ii) cellular network 111 supported by mobile switching center 110. The enterprise network supported by private branch exchange 105 provides telecommunications service to one or more telecommunications terminals, for example terminals 106 and 107, within the enterprise area served, such as an office building or campus. Cellular network 111, which is supported by mobile switching center 110, provides telecommunications service to one or more cellular telecommunications terminals, including terminals 112 and 113.
Additionally, network 101 provides telecommunications service to other telecommunications terminals, such as terminals 102 and 103. For example, terminal 102 or 103 might originate a call that routes through network 101 to private branch exchange 105.
Mobile switching center 110 is capable of switching incoming calls from network 101 to cellular-capable terminals that have registered with switching center 110, such as terminals 112 and 113. Switching center 110 is also capable of handling outgoing calls from cellular-capable terminals to network 101. Switching center 110 communicates with terminals 112 and 113 via one or more radio base stations in cellular network 111.
Private branch exchange (PBX) 105 is capable of switching incoming calls (e.g., from terminal 102, etc.) from telecommunications network 101 via one or more communications paths to one or more on-premises terminals, such as on-premises terminals 106 and 107. Private branch exchange 105 is also capable of handling outgoing calls from on-premises terminals to network 101 via one or more communications paths.
Private branch exchange 105 is also capable of forwarding an incoming call, such as from terminal 102, to a telephone number of a PBX user's “off-premises” terminal that is accessible through network 101. This type of forwarding to a terminal affiliated with exchange 105 is also known as “extending” a call because the connection to the off-premises terminal appears to exchange 105 as an additional PBX line. Exchange 105 extends the call to the off-premises terminal in addition to switching the same incoming call to an “on-premises terminal” within the enterprise area that exchange 105 serves. In telecommunications system 100, terminals 106 and 107 are on-premises terminals with respect to private branch exchange 105, while terminals 103 and 112 are off-premises terminals with respect to exchange 105. Note that in system 100, terminals 102 and 113 are not off-premises terminals because, unlike terminals 103 and 112, they are not affiliated with exchange 105.
To accomplish (i) the switching of an incoming, enterprise-related call to an on-premises terminal and (ii) the extending of the call to the correct off-premises terminal, private branch exchange 105 maintains a table that correlates the off-premises telephone number to the on-premises, private branch exchange extension. Table 1 depicts a table that illustrates the correlation.
TABLE 1PBX Extension-to-PSTN Number DatabaseOn-PremisesTelecommunicationsPrivate BranchTelecommunicationsTerminalExchange ExtensionNetwork Number106732-555-0102, x11201-555-1236107732-555-0102, x12908-555-3381. . .. . .. . .
As an example, a first caller at terminal 102 who wishes to reach the PBX user of terminal 106 dials the PBX number (i.e., 732-555-0102). Private branch exchange 105 receives the incoming call and the extension number (i.e., x11) as specified by the caller. By using stored information that is similar to the information in Table 1, private branch exchange 105 determines that the call is also to be extended to off-premises telephone number 201-555-1236. This is the telephone number that is associated with off-premises terminal 112, which belongs to the PBX user of terminal 106. Exchange 105 then extends the call to terminal 112. The idea behind transmitting the call to both terminals 106 and 112 (or sometimes to terminal 112 only) is that if the PBX user is not reachable at his office phone (i.e., terminal 106), then possibly he is reachable at a phone that is outside of the office (i.e., terminal 112). The extending of the call to an off-premises terminal enhances the caller's experience by only requiring the caller to use a single telephone number to reach the PBX user, regardless of whether or not the PBX user is in the office.
Continuing with the example, a second caller at terminal 113, who also wishes to reach the PBX user of terminal 106, dials the same PBX number (i.e., 732-555-0102). Private branch exchange 105 receives the second incoming call and the extension number (i.e., x11) as specified by the caller, and determines that terminal 112 is the intended recipient for the second call also. Terminal 112, however, is already handling the first call; consequently, exchange 105 holds the second call.
At this point in the example, private branch exchange 105 is holding one call for off-premises terminal 112. Furthermore, if one or more additional calls arrive with terminal 112 as the intended terminal, private branch exchange 105 will have to maintain multiple held calls for terminal 112, as well as any calls that are being held for other off-premises terminals, such as terminal 103. The user of the off-premises terminal, which is typically an ordinary cell phone or wireline telephone, cannot easily keep track of various held-call characteristics, such as: (i) whether there are calls still on hold and (ii) if there are held calls, the number of calls on hold. Furthermore, a backlog of held calls can become overwhelming for the user to deal with. What is needed is a technique to manage held calls, without some of the disadvantages of the prior art.